A planet as hot as the sun

To date, Kepler is by far the most appropriate telescope for studying these stars. One of the first stars studied by the telescope immediately revealed a surprise. “As the photometric data relative to this star reached us from Kepler, two very low frequencies appeared more and more clearly. They were much too weak to correspond with the pulsation frequencies of the star. We therefore envisaged all possible interpretations. The only plausible one was the presence of two exoplanets orbiting the remains of this red giant, with periods of 20 750 and 29 650 seconds. Therefore these planets orbit their star in 5.76 and 8.23 hours respectively, while our planet takes one year”, explains Valérie Van Grootel. These were the first exoplanets to be found by this photometric method.

Simultaneous observation of the star and its exoplanets makes it possible to characterize the latter: these are planets that are very close to their sun and very small, typically around the size of the Earth. But they are totally different to our blue oasis: the exoplanets are more like hell. Judge for yourselves: a temperature of no less than 6000°C reigns on their surfaces, in other words, the same temperature as our sun. Our researcher continues, “All this seems incredible and it all stems from two small frequencies that appeared in the spectrum of their star, which could only be explained by a bodies gravitating around it. At this temperature, only iron and nickel remain: the other elements must have already evaporated a long time ago. How long will these planets survive”? The destiny of such a planet could be to collapse into its star or evaporation (see article “An exoplanetary drama”).

This is not the only extraordinary fact here. Twenty million years ago (only), the star was still a red giant. Its planets gravitated inside the star which had a very weak envelope. Since then, the red giant has discarded its envelope to reveal its planets. This observation by Kepler could form the key to understanding the physical mechanism at work in the ejection of the envelope of a red giant. Indeed, the majority of theoretical models bring into play a stellar companion to the red giant.  When the latter is not part of a couple, it is difficult to explain the loss of the envelope. The presence of planets could compensate for the loss lack of  a stellar companion. Indeed, the planet, attracted by the more massive core of the star will migrate towards the centre. By this movement, it releases energy into the star’s envelope which will accumulate and contribute to the release of the envelope. “In fact we think that the two small exoplanets that we have detected are really former giant planets whose gaseous envelopes evaporated during the immersion phase, at the same time as the star discarded its own envelope. So a core of a star and two planet cores remain. Simulations are being carried out. But the idea seems to be watertight”,   continues Valérie Van Grootel.

Until a short time ago, extreme horizontal branch stars were considered to be insignificant due to their very small size, their weakness and the fact that they are so rare. Those who studied them were not taken seriously. However, Kepler and CoRoT have brought these stars into the limelight, along with the red giants which had also been ignored for quite a long time (see article “Imposing Giants”.) The tide is turning.

Core Giant planet

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